There are three carbon isotopes that occur naturally in living matter: .sup.12 C which makes up approximately 99 percent of naturally occurring carbon; .sup.13 C which makes up approximately 1 percent of naturally occurring carbon; and radioactive .sup.14 C which has a natural abundance in parts per trillion.
In 1946 Willard Libby developed a technique of age determination using radioactive .sup.14 C. .sup.14 C is constantly being generated through the interaction of cosmic rays with the upper atmosphere at the rate of approximately 7.5 kilograms of .sup.14 C per year. This radioactive carbon, in the form of CO.sub.2 enters the "carbon cycle" as plants utilize CO.sub.2 in the production of food. .sup.14 C is constantly decaying with a half-life of approximately 5730 years. In the atmosphere where .sup.14 C is constantly replenished, there is a constant ratio between the amount of .sup.14 C and the isotopes of carbon. Thus, in all living matter, which is constantly interacting with the atmosphere through the carbon cycle, the ratio of .sup.14 C to the other carbon isotopes remains essentially constant.
When a living organism dies, the organic matter which forms the organism no longer interacts with the atmosphere. Thus, over time, as the radioactive C.sup.14 decays, the ratio between .sup.14 C and the non-radioactive isotopes of carbon changes and this change in the ratio can be used to determine how long an assemblage of organic material has been dead.
The technique, as originally practiced, determined the concentration of .sup.14 C by monitoring the radioactive beta decay of the .sup.14 C. The .sup.14 C activity in living plants and animals and in the air is approximately fourteen disintegrations per minute per gram of carbon.
Monitoring of the beta decay of .sup.14 C in a sample of dead organic matter allows the calculation of the amount of time which has passed since the plant or animal from which the organic matter is derived died.
Monitoring the decay of .sup.14 C is an effective method of dating organic matter but the process has several limitations which, in many situations, can render the process inaccurate or impractical. Because of the relatively low radioactivity of normal inorganic matter, rather large samples, on the order of a gram, are required for the conventional process. These samples must be destroyed during the dating process and thus a significant portion of a test sample may have to be sacrificed in order to use the .sup.14 C method. Further, because radioactive decay is a quantum mechanical phenomenon, the number of beta decays which a carbon sample emits will vary considerably from minute to minute and thus the sample must be monitored over a considerable period of time in order to achieve a statistically meaningful value for the amount of .sup.14 C present.
The smaller the sample, the greater the amount of time required to determine the amount of .sup.14 C present in the test sample by monitoring beta particle emission. At the same time, the older the sample the more sensitive the age-determination is to a precise measurement of the .sup.14 C present, while at the same time, the lower overall activity of the sample. These physical limitations are combined with the practical limitations that many ancient artifacts in which there is interest in dating, such as the Shroud of Turin, are of such value that only extremely small samples can, in the interest of preservation, be submitted for destruction in the carbon-14 dating process.
The foregoing physical and practical limitations mean that .sup.14 C dating by conventional methods is of limited practicality when the sample size available is small, the material to be dated is very old, the object to be dated is of great historical value, or a large number of objects must be dated to produce useful information. Additionally, where the carbon sample to be dated has become contaminated with modem carbon, it may be possible to find individual grains of carbonaceous material, which, if they could be tested, could be shown to be either modem or of the ancient material of interest.
To overcome these problems, methods and apparatus have been developed for utilizing a mass spectrometer for determining the amount of .sup.14 C present in a sample, directly. Because all the .sup.14 C atoms in a sample are available for detection, the ratio between .sup.14 C and the other isotopes in a carbon sample can be rapidly determined for extremely small samples with a high degree of precision.
There are a number of practical difficulties associated with the use of a mass spectrometer to determine the isotopic ratios between .sup.14 C and the other isotopes of carbon.
The measurement of the ratio of .sup.14 C to .sup.12 C is accomplished with the aide of a tandem accelerator. First, negative ions of carbon are formed from the sample to be tested. The sample may be as small as approximately 1 milligram of carbon. The negative ions are analyzed in a low energy negative ion mass spectrometer. They are then supplied to a tandem accelerator where they are accelerated to a few million electron volts so that molecular ions, such as .sup.12 CH.sub.2 -- and .sup.13 CH--, can be eliminated. This elimination is accomplished by the passage of the ions through a long tube of high pressure gas, known as a stripping canal, located in the center of the tandem accelerator where the carbon ions lose four electrons to become triply charged positive ions. These ions are then accelerated further by the accelerator and then analyzed by another mass spectrometer system prior to the counting of the individual ions.
Each isotope of carbon forms part of the beam current produced by the accelerator. The.sup.12 C makes up 99 percent of naturally occurring carbon. To first approximation the .sup.12 C portion of the beam current produces 99 percent of the x-radiation and neutrons produced during the acceleration of the carbon isotopes.
U.S. Pat. No. 5,013,923 to Litherland, et al. discloses an apparatus which is placed before the tandem accelerator which separates the isotopes of carbon and any contaminating isomers into separate, physically spaced-apart beams, and then recombines the beams for injection into the tandem accelerator.
The recombinator described in the Litherland, et al. patent serves the stated function of the elimination of atoms and molecules of other masses by means of apertures at the midpoint of the recombinator.
Though not stated in the Litherland, et al. patent, the Litherland device is normally employed with a beam chopper which sequentially in time prevents the .sup.12 C ion beam from entering the tandem accelerator. The function of the beam chopper is to attenuate the .sup.12 C current so as to reduce the x-radiation and neutrons produced during the acceleration of the carbon isotopes.
Because the efficiency and function of the entire carbon-14 dating apparatus can fluctuate over a relatively short time period and because the precision of the ultimate measurement of .sup.14 C must be made with respect to the amount of .sup.12 C present, it is desirable that the measurements of the amount of .sup.12 C, .sup.13 C and .sup.14 C be simultaneously or/and closely sequenced in time, which is accomplished by the rapidly rotating beam chopper.
Southon, et al. describe, in their article, "Injection system for AMS: Simultaneous v Sequential," published in Nuclear Instruments and Methods in Physics Research, B52(1990), 370-374 North-Holland, another apparatus for separating an ion beam composed of .sup.12 C, .sup.13 C and .sup.14 C into spatially parallel beams where one or more of the beams may be eliminated before the beams are combined and injected into a tandem accelerator. The device described in the Southon, et al. paper, while functional, suffers from a difficulty in tuning. Southon, et al. describes the process of tuning as being "repeated until the operator was satisfied or his spirit was broken."
What is needed is an apparatus for allowing the sequential and simultaneous injection of carbon isotopes into a tandem accelerator for determining isotope ratios which is easy to tune.